Add Maven Wrapper 3.3.2 with Maven 3.9.6

Add Maven Wrapper 3.3.2 with Maven 3.9.6

Adds the standard Maven Wrapper (JAR-less, Apache Maven Wrapper 3.3.2)
so that the project can be built without a pre-installed Maven.
The wrapper downloads Maven 3.9.6 on first use and caches it locally.

- .mvn/wrapper/maven-wrapper.properties: distribution URL for 3.9.6
- mvnw: Unix startup script (executable)
- mvnw.cmd: Windows startup script
- CLAUDE.md: document wrapper usage, version update procedure, and
  the new rule to verify the build after each commit

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>